Bipolar disorder and borderline personality disorders are both characterized by unstable moods and impulsive actions, but they are different diagnoses, and have different recommended treatments.

Bipolar disorder and borderline personality disorder often get confused with each other. I often see patients who tell me a family member has been diagnosed with bipolar disorder, and when we review the DSM criteria for bipolar disorder and borderline personality disorder, it appears that borderline personality disorder better explains their family member.

A widely-used screening tool for the diagnosis of bipolar disorder may actually be identifying borderline personality disorder.

In the article that appears online in the Journal of Clinical Psychiatry, the researchers question the effectiveness of the Mood Disorder Questionnaire (MDQ). The MDQ is the most widely-used and studied screening tool for bipolar disorder. It is a brief questionnaire that assesses whether a patient displays some of the characteristic behaviors of bipolar disorder.
The study consisted of the following:

The research team interviewed nearly 500 patients using the Structured Clinical Interview for Diagnostic Statistical Manual IV (DSM-IV) and the Structured Interview for DSM-IV for personality disorders.

The patients were also asked to complete the MDQ.

After scoring, the researchers found that:

Patients with a positive indication for bipolar disorder using the MDQ were as likely to be diagnosed with borderline personality disorder as bipolar disorder when using the structured clinical interview.

Borderline personality disorder was four times more frequently diagnosed in the group who screened positive on the MDQ.

While only a qualified clinician can make a diagnosis bipolar disorder or borderline personality disorder, there are some differences that clinicians use to make this determination:

Persons with bipolar disorder will have episodes of depression, and episodes of euphoria or irritability, and even periods of normal mood in between. There are other associated symptoms that go along with the mood changes, such as low energy, and behavioral changes.

People with bipolar disorder will cycle between these mood states over months to even years. Rarely, the mood states change more rapidly. But in borderline personality disorder, the shifts in mood are far more rapid—even several times a day.

People with borderline personality disorder are more affected by what is happening in their life at any moment, and how they feel about it, and they react to it. They are particularly sensitive to abandonment, or fears of abandonment.

The ups and downs in borderline personality disorder are not all-encompassing mood shifts of mania and depression. Instead, they have specific feelings that fluctuate: fear, anger, sadness, disgust, love, in a sort of all-or-nothing, black and white way.